This invention relates to a method of increasing the efficiency of introducing material into a furnace for treatment. Specifically, the invention relates to a method of decreasing the amount of ambient air entering a furnace.
A rotary hearth furnace is a continuous heating furnace generally having a refractory roof supported by a ring-shaped inner refractory wall that is surrounded by a ring-shaped outer refractory lined wall. The inner wall and the outer wall form a circular hearth path along which a rotating hearth travels. The rotating hearth generally consists of a support plate including a top of refractory material and having side elevations. The rotary hearth furnace is heated by a plurality of burners placed around the path of travel of the rotating hearth.
Feed material to be treated in the furnace is typically loaded, or dropped, from a feed material charger through gaps in the roof of the furnace. A feed material charger for use in a rotary hearth furnace is described in U.S. Pat. No. 5,895,215 and incorporated herein by reference.
Iron oxide and other metallic oxide material can be reduced by a direct reduction processes. A direct reduction process includes heating the metallic oxide material, particularly when it is in a pelletized or briquetted form. Metallic oxides which can be reduced are principally of the iron group, i.e., iron, cobalt and nickel oxides. These oxides and the metals themselves have a high melting temperature and are readily reduced using carbonaceous reductants. Preferably the reductant for these oxides is finely divided coal or coke used in an amount somewhat in excess of the stoichiometric amount required for reduction. Heating can by accomplished by introducing gaseous fueled flames to a temperature within the range of from about 1800 degrees F. to about 2600 degrees F. The heating can be done in a furnace, such as a rotary hearth furnace.
A rotary hearth furnace includes a slowly moving rotary hearth, upon which the material to be heated rests. The material is introduced though an opening in the furnace at one or more points along the rotary hearth""s trajectory. Before the rotary hearth is able to complete one revolution, the material is treated and then removed from the rotary hearth""s surface. While the rotary hearth is rotating, the metallic oxide material is exposed to heat. The heat can be in the form of a multiplicity of burners.
A number of challenges are encountered in reducing metallic oxide material in rotary hearth furnaces. One challenge is the presence of excess oxygen in the furnace. Since ambient air contains oxygen, it is desirous to minimize the amount of ambient air in the rotary hearth furnace. Rotary hearth furnaces are typically operated such that the interior of the furnace is at a negative pressure. Uncontrolled conditions inside the furnace increases the negative pressure, drawing ambient air into the furnace.
Another challenge is maintaining a relatively constant time versus temperature exposure of the metallic oxides to the heat source. A relatively uniform reduction of metallic oxide requires a relatively uniform exposure of the metallic oxide material to the heating source over a given time. An even distribution of metallic oxide material onto the rotary hearth provides a superior reduction product. The depth of metallic oxide material loaded onto the rotary hearth should be as uniform as possible.
Uniformity in the reduction product can also be facilitated by maintaining uniformity in size and shape of the metallic oxide material. Typically, the metallic oxide material is in the form of roughly spherical pellets within the range of from about 1 to about 3 cm. in diameter. Alternatively other shapes of agglomerates can also be used, for example, brick, briquette, or other suitable shapes.
The above objects as well as other objects not specifically enumerated are achieved by an apparatus for charging feed material through an opening into a furnace having a fixed structure oriented at an acute angle to the vertical and a moveable door. The moveable door together with the fixed structure defines the opening. The moveable door is mounted for movement toward and away from the fixed structure to widen and narrow the opening. The moveable door is biased toward the fixed structure. The moveable door and the fixed structure create a V-shaped downwardly pointing channel to direct the feed material being fed through the opening by gravity.
According to this invention there is also provided an apparatus for charging feed material into a rotary furnace comprising a rotary furnace roof, a feed material charger and an opening in the rotary hearth roof. The feed material charger is positioned above the rotary furnace roof for charging the feed material to the rotary furnace. The opening in the rotary hearth roof accepts the feed material into the rotary furnace. The opening is defined by a series of moveable doors together with the fixed structure. The moveable doors are mounted for movement toward and away from the fixed structure to widen and narrow the opening. The moveable door is biased toward the fixed structure by a counterweight.
According to this invention there is also provided an apparatus for charging feed material through an opening into a furnace having a first moveable door and a second moveable door which, together with the first moveable door, defines the opening. The first moveable door and second moveable door are biased toward each other.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.